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MODELING
Helicopter timer model. Tips for a modeller
Directory / Radio control equipment In aircraft modeling circles of the House of Pioneers of the Kominternovsky District, at the regional SUT and at the Kharkov secondary school No. 112, work has been underway on coaxial models of helicopters for several years. Their designs are different, but they all fly steadily. The model described below is one of the latest modifications. At the competition "Experiment-78" in the class of timed models of helicopters, she took first place. The model (Fig. 1) is made according to the coaxial scheme with the lower location of the KMD-2,5 engine. Traction screw Ø 250 mm and pitch 70 mm. The flight weight of the model is 770 g. The chassis of the model is tricycle. The front strut made of bamboo Ø 4-5 mm is glued with resin into the forward part of the fuselage. Rear - made of 1 mm thick duralumin, attached to the tail boom, on which an imitation of a turbine plant made of balsa plate is glued.
Rice. 1. Scheme of the helicopter model: 1 - body tube (Ø 22x15), 2 - rotor shaft, 3 - rotor hub, 4 - axial hinge of the blade, 5 - blade angle limiter, 6 - blade, 7 - counterweight, 8 - cable, 9 - plain bearing, 10 - tank motor mount, 11 - engine with propeller, 12 - engine counterweight, 13 - chassis, 14 - triangular plate, 15 - clamp. The rotor is three-bladed. Blade profile B-6308, coordinates are given in the table. Coordinate table Blades 6 - from balsa. The front and rear edges are made of 2 mm thick basswood. The surface is carefully processed, pasted over with glass cloth 0,025 mm thick on liquid epoxy resin and painted with pentaphthalic paint. A lime spar of variable cross section runs along the entire blade, fading to nothing at the end. A steel axle Ø 4 mm is glued into its root part for mounting the blade in the hub. In places where the axis is glued and counterweights are installed, pads made of plywood 1 mm thick are glued to the top and bottom of the blade. Counterweights 7 - made of plywood 2 mm thick, varnished AK-20 and painted with pentaphthalic paint. In their root part, two M2 studs are glued for attachment to the blade. Weights weighing 10 g are suspended from the counterweight by means of a screw and an M3 nut. The rods from them from a cable Ø 0,3 mm go to the rotor shaft, made of a duralumin tube Ø 9,9, 240 mm long. The cables 8 are attached with a clamp 15 and a triangular plate 14. All parts of the rotor hub assembly are made of D16T duralumin. Sleeve 3 itself is machined on a lathe. This method of its manufacture allows you to very accurately fix the angle of the "tulip" of the blades (4°) and the angle between them (120°). Note that the exact fixation of the angles plays a very important role when adjusting the model. Limiters 5 serve to adjust the installation angles of the blades during motor flight and in autorotation mode.
The assembly of the attachment points of the blade and the sleeve 3 of the rotor is carried out as follows. Rods 7 (see Fig. 2) are inserted into the sleeve 1 of the rotor. In this case, the holes in them are combined. Pins 3 are inserted into holes Ø 2 mm, which are fixed with cotter pins Ø 0,8 mm on both sides. These cotter pins are cut off when the blade hits the ground, thereby preserving the propeller. A bushing 8 is put on the axle 9, which is held with a washer and a pin so that the axial clearance ensures free rotation of the blades 11. The bushings 9 are inserted into the rod 7 and pulled together by the collet clamp of the limiter 10, which is previously put on the rod. Then, with the help of M2 screws in the paws of the limiter, the angles of attack of the blades are set in the range from +22 to -7 °. This position of the limiter is finally fixed. The assembled bushing is attached to the model shaft with a collet clamp. Currently, requirements are imposed on helicopters as semi-copies with a certain midsection, so the fuselage is made with a three-dimensional cabin, which, together with the canopy, is glued from fiberglass. The nose of the fuselage is curved from plywood 1 mm thick. The tail boom is made of balsa wood with pine piping top and bottom. At the end there is a keel in the form of a ring Ø 120 mm, covered with a transparent film. The nose and tail boom are attached to the body tube 1 (see Fig. 1) with six M3 screws. To reduce the flight weight of the model, sliding bearings 9 are glued into the body tube at both ends of it, through which the rotor shaft 2 of a duralumin tube passes (outer Ø 9,9 mm, wall thickness 1 mm). On one side, a sleeve 3 of the rotor is put on it, and on the other, a tank motor mount 10. Of great interest to modellers is the tank mount. Two variants of its design were developed. The earlier one is option 1, the last one is option 2. Both designs are made on a lathe from D16T duralumin, and individual elements (flange, grooves for the needle and jet, collet) were finalized manually. These designs made it possible to abandon the separate manufacture of the engine mount and tank, which reduced the weight of the helicopter. In addition, such a tank motor mount allows you to quickly assemble and disassemble the model. As can be seen from the figure, the tank-motor mount "Variant-1" was attached to the shaft with a screw passing through. Such a scheme turned out to be not the best, since the through hole on the shaft concentrated stresses and, upon impact, the shaft broke at the drilling site. The design of the "Option-2" tank has a collet-type clamp, which further reduces the assembly and disassembly time of the model. Air is taken in by the engine through the hollow shaft of the rotor, the end of which rests against the diffuser inside the tank. In the manufacture and installation of the tank-motor mount, great attention should be paid to its centering relative to the engine shaft, since any displacement creates a significant beating and impairs the flight of the device. In the "Variant-1" tank, the lid is fixed with six M2 screws through a gasket, and in the "Variant-2" tank, a glass was screwed on, its thread was previously lubricated with epoxy. Tank fittings are machined from steel and glued on epoxy resin. Filling and drainage are installed as close as possible to the rotor shaft, on top of the tank, feeding - as close as possible to its outer side surface, on the bottom. Such an arrangement of fittings ensures an uninterrupted supply of fuel and eliminates its leakage during the rotation of the tank-motor mount. The latter is attached with a rectangular flange 28x28 mm to the rear wall of the engine crankcase. Holes for the jet and needle are marked, which are drilled and finished with a needle file to the required shape. Author: V.Dvorkin
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